In recent years, there has been a growing interest in environmental issues and CO2 output regulation has become severe. In the field of automobiles, an improvement in fuel efficiency through car body weight reduction has been a large issue. Consequently, reduction in thickness through application of high strength steel sheets to automotive parts has been pursued and steel sheets having tensile strength TS of 590 MPa or more have been applied.
The high strength steel sheet used for structural members and reinforcing members of automobiles is required to have excellent elongation and stretch-flange-formability. In particular, a high strength steel sheet used to form parts having complicated shapes is required to have both excellent elongation and stretch-flange-formability rather than a single characteristic of them. In addition, it may take time (elapsed time) from production of the high strength steel sheet until the steel sheet concerned is actually subjected to press forming, and it is an important characteristic of the high strength steel sheet that elongation is not degraded because of aging in this elapsed time.
Also, the high strength steel sheet used for structural members and reinforcing members of automobiles is required to have high dimensional accuracy because the high strength steel sheet is press-formed and, thereafter, assembled and modularized by arc welding, spot welding or the like. Therefore, it is necessary that spring-back and the like of such a high strength steel sheet does not occur after forming so that a low yield ratio is required before forming. In this regard, the yield ratio (YR) is a value indicating the ratio of yield strength (YS) to tensile strength (TS) and is represented by YR (%)=(YS/TS)×100(%).
A dual phase steel (DP steel) having a ferrite-martensite multi-phase is known as a high strength steel sheet with a low yield ratio, having formability and high strength in combination. The DP steel is a multi-phase steel in which martensite is dispersed in ferrite serving as a main phase and has high TS, a low yield ratio, and an excellent elongation characteristic. However, the DP steel has a disadvantage that the stretch-flange-formability is poor, because cracking easily occurs owing to concentration of applied force at the interface between ferrite and martensite.
Then, for example, technologies of Japanese Patent No. 3936440 and Japanese Unexamined Patent Application Publication No. 2008-297609 have been proposed to allow even the DP steel to have excellent stretch-flange-formability. Japanese Patent No. 3936440 discloses an automotive high strength steel sheet, where the space factors of ferrite and martensite relative to the entire microstructure and the average grain sizes thereof are controlled and fine martensite is dispersed in a steel so that degradation of the stretch-flange-formability is suppressed and, thereby, both collision safety and formability are ensured. Japanese Unexamined Patent Application Publication No. 2008-297609 discloses a high strength steel sheet, where elongation and stretch-flange-formability of a multi-phase steel sheet mainly including a ferrite phase and a martensite phase are improved by controlling the space factors of fine ferrite having an average grain size of 3 μm or less and martensite having an average grain size of 6 μm or less relative to the entire microstructure.
In addition, a TRIP steel sheet (transformation induced plasticity) is mentioned as a steel sheet having high strength and excellent ductility in combination. The TRIP steel sheet includes retained austenite in the steel sheet microstructure thereof. When the TRIP steel sheet is subjected to forming at a temperature higher than or equal to the martensite transformation start temperature, large elongation is obtained through stress induced transformation of retained austenite into martensite. However, in this TRIP steel sheet, retained austenite is transformed into martensite during blanking and, thereby, cracking occurs at the interface with ferrite. Consequently, the TRIP steel sheet has a disadvantage that the stretch-flange-formability is poor.
Then, a technology has been proposed, wherein even the TRIP steel sheet is provided with excellent stretch-flange-formability in addition to excellent ductility (elongation). For example, Japanese Patent No. 3508657 discloses a high strength cold rolled steel sheet exhibiting improved stretch-flange-formability and having a multi-phase composed of ferrite, retained austenite, and a phase generated at low temperature. Japanese Patent No. 3508657 discloses that the stretch-flange-formability is improved by making the ferrite grain size fine through addition of an appropriate amount of Ti and controlling the shape of sulfide based inclusions through addition of Ca and/or REM. Also, Japanese Patent No. 4288364 discloses a multi-phase cold rolled steel sheet having a multi-phase including ferrite, retained austenite, and the remainder composed of bainite and martensite and having excellent elongation and stretch-flange-formability. Japanese Patent No. 4288364 discloses that the aspect ratios and average grain sizes of martensite and retained austenite are specified and, in addition, the numbers per unit area of martensite and retained austenite are specified.
On the other hand, when a part having a particularly complicated shape is press-formed by using the above-described high strength steel sheet having TS of 590 MPa or more, further reduction in YR is required and, in addition, excellent elongation and stretch-flange-formability are required. For example, a steel sheet having a tensile strength (TS) of 590 MPa or more and a yield ratio (YR) of 64% or less, where the hole expansion ratio serving as an index of the stretch-flange-formability of 60% or more and the elongation (total elongation) of 31% or more can be ensured, has been desired.
However, conventional high strength steel sheets cannot sufficiently satisfy such characteristics. For example, in Japanese Patent No. 3936440, enough stretch-flange-formability for press forming cannot be ensured, although the average grain sizes of ferrite and martensite of the steel sheet are specified. Japanese Unexamined Patent Application Publication No. 2008-297609 has a problem that the volume fraction of martensite in the resulting steel sheet is considerably large and, thereby, elongation is insufficient in relation to strength. Japanese Patent No. 3508657 and Japanese Patent No. 4288364 have the problem that YR of the resulting steel sheet is high and, thereby, spring-back and the like occur easily after forming. As described above, in the actual situation of conventional high strength steel sheets, a steel sheet which has achieved the above-described high strength and low yield ratio and which has excellent elongation and stretch-flange-formability in combination has not been developed.
It could therefore be helpful to provide a high strength steel sheet having excellent elongation, excellent stretch-flange-formability, and a low yield ratio and a method of manufacturing the same. Specifically, a high strength steel sheet with a low yield ratio and a method of manufacturing the same are needed, where the yield ratio (YR)≤64% and the tensile strength (TS)≥590 MPa are satisfied so that the hole expansion ratio (λ)≥60% and the total elongation (EL)≥31% can be ensured.